Well, the answer might be that the sound clock we are talking about is not a true sound clock. Yes, the pings are the result of sound waves travelling.
But the rest of the clock is made of material OTHER than sound waves.
If the clock was ENTIRELY made of sound waves (however that might happen) then perhaps you would get a similar length contraction to the light clock.
So in comparing the two clocks, we're not actually comparing like for like.
There is no basis in theory to propose that sound undergoes length contraction, and
A sound clock made up completely of sound is a ridiculous suggestion. And it's not like for like, since a light clock is not comprised completely of light. You have a source, a mirror, and a detector.